1. Field of the Invention
The present invention relates to a developing device, a process cartridge and an image forming apparatus loaded therewith and more particularly to charge control effected with a two-ingredient type developer, i.e., a toner and carrier mixture.
2. Description of the Background Art
Generally, in an electrophotographic image forming apparatus, charging means, exposing means, developing means, image transferring means, separating means, cleaning means and discharging means are arranged around an image carrier implemented by a photoconductive element. To meet the increasing demand for easy maintenance of such processing means, it has been proposed to construct part of them into a single process cartridge in, e.g., Japanese Patent Laid-Open Patent Publication No. 2001-83862.
On the other hand, it is a common practice with a developing device to use a two-ingredient type developer or toner and carrier mixture. Toner grains and carrier grains, constituting the developer, are mixed together and electrified thereby before development, so that the toner grains are charged and then deposited on a latent image formed on the image carrier. While the two-ingredient type developer is usually mixed and agitated in the developing device, it is also ready to be done so when a process cartridge, including the developing device, is mounted to an image forming apparatus.
In an image forming apparatus of the type capable of forming a full-color image by superposing images of different colors, toner grains and carrier grains are also mixed and agitated in a developing device arranged in the apparatus. In this type of image forming apparatus, a particular process cartridge is assigned to each color and replaced independently of the others when its life ends. However, preparing a particular process cartridge for each color not only increases management costs at the user's used, but also increases costs at the manufacturer's side including parts cost and other machining costs.
In light of the above, Laid-Open Publication No. 2001-83862 mentioned above, for example, teaches process cartridges having an identical configuration for thereby making it unnecessary to produce a particular process cartridge for each color.
On the other hand, when toner grains and carrier grains are electrified by being mixed together, a charging characteristic varies in dependence on environmental conditions, particularly humidity, in a portion where the different kinds of grains are mixed, as disclosed in, e.g., Japanese Patent Laid-Open Publication No. 9-146360. This is also true when process cartridges are used.
While process cartridges identical in structure are successful to reduce the kinds of process cartridges required and therefore costs, they have the following problems left unsolved. A developer stored in each process cartridge is of a particular color, so that each process cartridge must be mounted to a particular position on an image forming apparatus. Therefore, even if the process cartridges are provided with the same structure, there must be prepared process cartridges each storing a developer of a particular color. This fails to substantially reduce the kinds of process cartridges and therefore management costs.
Further, when fresh toner is replenished to a conventional process cartridge, a period of time necessary for a preselected charging characteristic to be established is extended, delaying the start or the restart of image formation. Particularly, because an increase or a decrease in the amount of charge to be deposited on toner is susceptible to humidity, it is likely that the above period of time is extended by the influence of humidity in the mixing and agitating space also. Moreover, when part of the developer is used for development in a defectively electrified condition, toner is apt to fly about due to weak adhesion to a latent image and smear the inside of the apparatus.